1. Field of the Invention
The present invention relates to a multi-layer optical recording medium such as a video disk or a compact disk, and more particularly to, a multi-layer optical recording medium in which a reflectivity of a first reflecting layer is improved.
2. Description of the Related Art
FIG. 1 is a schematic illustration of the structure of a conventional optical disk. The conventional optical disk is made of a transparent resin material such as polymethylmethacrylate (PMMA) or polycarbonate (PC) and has a stacked structure of a transparent substrate 10 having a predetermined pit pattern, a reflecting film 12 formed by depositing a metal using an evaporation method, such as sputtering, on transparent substrate 10, and a protecting film 14 formed on reflecting film 12. The pit patterns of reflecting film 12 and transparent substrate 10 have the same shape. Information, i.e., the pit pattern formed in reflecting film 12, is read by an optical device in a known manner.
However, the conventional optical disk structure described above has a drawback in that the capacity of the optical disk is insufficient to store information requiring a larger density, such as video and audio information. The pit pattern on the disk is formed in a known manner to correspond to recorded information. For example, the pit pattern can be formed by press molding or by known ultraviolet techniques.
Accordingly, multi-layer optical disk structures are used to create a larger storage capacity. A multi-layer optical disk structure (U.S. Pat. No. 4,450,553 by Phillips) having reflecting layers of which reflecting coefficients are different from each other and a multi-layer optical disk structure (U.S. Pat. No. 5,126,996 by Pioneer) with silicon as the first reflecting layer and aluminum as the second reflecting layer have been proposed. These multi-layer optical disk structures will be explained with reference to FIGS. 2 and 3.
FIG. 2 shows the structure of the Phillips optical disk. An optical disk includes a transparent substrate 20 made of a thick PVC plate of about 1 mm, a first resin layer 22 formed on transparent substrate 20 and processed by ultraviolet rays, a first reflecting film 24 formed on first resin layer 22, a first adhesion layer 26 formed on first reflecting film 24, a spacing sheet 28 formed on first adhesion layer 26 and made of a thick PVC plate of about 0.15 mm, a second adhesion layer 30 formed on the spacing sheet 28, a second reflecting film 32 formed on second adhesion layer 31, a second resin layer 34 formed on second reflecting film 32 and processed by ultraviolet rays, and a transparent PVC plate 36 formed on second resin layer 34. First reflecting film 24 is adhered to spacing sheet 28 by first adhesion layer 26, and second reflecting film 32 is adhered to spacing sheet 28 by second adhesion layer 30.
The optical disk constructed as above has at least a two-layered reflecting optical structure. In each reflecting optical structure, the reflecting layers are constructed to have different reflecting coefficients with respect to each other. A stronger reflected light from the reflecting layers makes for easier reading of the information recorded on the reflecting film. Also, the ratio of incident light to reflected light depends on the number of layers in the optical structure. In the case of a two-layered reflecting optical structure as shown in FIG. 2, first reflecting film 24 is made of, for example, a dielectric material to transmit part of the radiated light beam (.lambda.) from the light source. In such a structure, the reflecting rate of a proper order is needed to read out precisely the information stored in first and second reflecting films 24 and 32. It is preferable that the reflecting rate of the first reflecting film 24 is 25-40%, and the reflecting rate of the second reflecting film 32 is 45-100%. Reflected light from the different layers is distinguished based on the intensity thereof to read properly the recorded information.
Since the above optical disk structure is formed to have more than two optical reflecting layers more than twice the conventional information storage capacity is attained in the same area. However, since the optical disk structure has a multi-layered structure composed of the resin layers, the adhesion layers, and the spacing layer, the manufacturing processes are difficult.
FIG. 3 shows the structure of the Pioneer optical disk. This multi-layer optical disk has a structure composed of a transparent substrate 50 made of a transparent resin such as PMMA or PC material as in the conventional case, a first reflecting film 52 formed on the transparent substrate, an adhesion layer 54 to adhere the first reflecting film 52 to a second reflecting film 56, a second reflecting film 56 adhered to the first reflecting film 52 by the adhesion layer, and a protecting film 58 formed on the second reflecting film 56. The first reflecting film 52 is made of silicon and the second reflecting film 56 is made of aluminum. These materials selectively reflect light depending on wavelength.
According to this structure, the first wavelength (.lambda..sub.1) of a beam radiated from a light source is reflected by the first reflecting film and the second wavelength (.lambda..sub.2) is transmitted. The second reflecting film reflects at least part of the second wavelength (.lambda..sub.2). When the information of the first reflecting film is read, the first wavelength (.lambda..sub.1) reflected by the first reflecting film is used. When the information of the second reflecting film is read, the second wavelength (.lambda..sub.2) transmitting the first reflecting film and reflected by the second reflecting film is used. Namely, in the above structure, light rays of a certain wavelength are transmitted through the first reflecting film (silicon layer) and reflected by the second reflecting film (aluminum layer). Accordingly, the information of each layer can be read by distinguishing the different wavelengths received by a detector.
The structure of the above optical disk is relatively simple. However, since the information of one layer is distinguished from information from another layer by wavelength and laser diodes used as a general light source generate a single wavelength, there must be a laser diode in the reading device for each of the layers. Thus, the reproducing device becomes complicated and expensive.